Layer coatings provided for solar management purposes are known in the art. Such coatings often seek to reflect infrared (IR) radiation while at the same time enabling a high amount of visible light transmittance. In the automobile industry, for example, vehicle windshields often must have a visible light transmittance of at least 70% in the United States, and at least 75% in Europe, even when laminated with a polyvinyl butyral (PVB) layer provided between opposing glass substrates. This need for high visible transmittance often conflicts with the need for good IR reflection, and it is difficult to obtain both simultaneously. Making low-E coating designs more difficult is the need for a mechanically and/or chemically durable coating, as well as the need for a coating with low visible reflectance (glass side) that does not suffer radical color change when viewed at different viewing angles. It is also desirable for such coatings to be heat treatable, so that they may be used in vehicle windshields where heat bending is required, tempered IG units, and the like.
U.S. Pat. No. 5,584,902 (commonly owned herewith) discloses a low-E coating system including, from the glass substrate outward, a stack of: Si3N4/NiCr/Ag/NiCr/Si3N4. Unfortunately, while this coating is heat treatable and low-E in nature, it is characterized by rather high emissivity and/or sheet resistance values which lead to rather low Rsolar (total solar energy reflectance) values around 22-24%. For example, one coating reported in the '902 patent had a sheet resistance (Rs) of 14.4 ohms/square and a normal emissivity (En) of 0.15 before heat treatment; and a Rs of 10.5 ohms/square and a En of 0.11 after heat treatment.
U.S. Pat. No. 5,557,462 (commonly owned herewith) discloses a dual silver low-E coating including from the glass outwardly a stack of: Si3N4/NiCr/Ag/NiCr/Si3N4/NiCr/Ag/NiCr/Si3N4. The coating system of the '462 patent is characterized by good emissivity values and good Rsolar values. Unfortunately, the coating, systems described in the '462 patent are not heat treatable (e.g., upon heat treatment Rs goes way up, such as from about 3-5 to well over 10, and haze tends to set in). Because the coatings described in the '462 patent are not heat treatable, as a practical matter they cannot be used in applications such as vehicle windshields (where heat bending is required) or other applications requiring heat treatment (HT) such as tempering, heat strengthening, or bending.
Certain other dual-silver coatings are described in the background section of the aforesaid provisional application. See also U.S. Pat. No. 6,060,178. Unfortunately, these coating systems are plagued by poor durability before and/or after heat treatment.